(a) Field of the Invention
The present invention relates to a photoreceptor for electrophotography having an over coating layer, and more in particular to the photoreceptor for electrophotography employed in a copying machine and a printer using an electrophotographic recording method.
(b) Description of the Related Art
Recently, organic photoconductive material has been widely employed as a photoreceptor for electrophotography because of its advantages such as width of material selection and high productivity. The photoreceptor for electrophotography employing the organic photoconductive material is utilized as a function-separating photosensitive material consisting of a charge generation layer and a charge transport layer layered with each other.
The photoreceptor for electrophotography is naturally required to have a specified sensitivity, electrical characteristics and optical characteristics depending on an electrophotographic process in which the above photoreceptor for electrophotography is employed. Since an electrical or mechanical force such as that produced in corona charging, toner development, transfer to paper and a cleaning treatment is directly applied on a surface layer of the photoreceptor for electrophotography which can be repeatedly employed, the surface layer is required to have high durabilities against the above force. Due to deterioration produced by ozone generating during the corona charging, the specific durabilities are required in connection with electrical characteristics against sensitivity decrease, potential decrease and residual potential increase or those in connection with mechanical characteristics against abrasion and scratches of the sensitive material generated by its sliding.
Conventionally, in order to elevate the mechanical durabilities, the employment of thermosetting or photo-setting resin as an over coating layer of the sensitive material is disclosed (for example, in Patent Publication No. JP-A-1996-160640, this disclosure will be hereinafter referred to as "Prior Art 1"). In accordance with Prior Art 1, a photoreceptor for electrophotography having excellent hardwearing properties and environmental resistances can be provided by employing a protective layer having electroconductive metal oxide particles dispersed in resin obtained by polymerization between a photo-setting acryl monomer and an oligomer, and a photo-polymerization initiator.
Various coating materials are also known which are applied to that other than the sensitive materials. For example, a curable composition having reactive silica and a polymerizable unsaturated group employed as a hard coating material such as a plastics optical component, a touch panel and glass is disclosed in Patent Publication No. JP-A-1997-100111 (Title of Invention: "Reactive Silica, its Preparation and Usage", this disclosure will be hereinafter referred to as "Prior Art 2").
A coating film having a resistance to scuffing, weatherability, adherence and curability and satisfying properties from transparence to translucence and from high glossiness to dullness can be formed on various substrates in accordance with Prior Art 2. These are useful especially for a protective coating having a resistance to scuffing and weatherability present on the surface of organic resin mold, and also useful for a coating material applicable to a plastic substrate of which a heat resistant property is poor.
However, the coating material employing the thermosetting over coating layer requires a thermal treatment at a high temperature for a long period of time, and this material cannot be used depending on a kind of an organic photoconductive material and a substrate material. Moreover, a further period of time of the thermal treatment is required for sufficiently curing the coating material in order to produce a hardness and a resistance to printing sufficient for the over coating layer and to stabilize the electrophoto characteristics resulting in the elevation of the manufacturing cost.
When the photo-setting over coating layer of Prior Art 1 is employed, a photo-isomerization reaction and a photo-decomposition reaction may occur in the organic photoconductive material by means of an ultraviolet ray which is necessary for the curing to lower the electrophoto characteristics.
In case of a contact development system in which toner is rubbed and adhered to the sensitive material at a development part to perform the development, the resistance to printing and the resistance to scuffing are insufficient if only the curable resin is employed in the over coating layer.
In Prior Art 1, the metal particles are dispersed to elevate the electrophoto characteristics, but the metal particles are likely to be peeled off by repeated developments, and the peeled-off part disadvantageously produces an image defect.
When, on the other hand, the coating material of Prior Art 2 which is highly resistant is employed as the over coating layer of the photoreceptor for electrophotography, the electrophoto characteristics are largely deteriorated not to put the material in practice.
In order to prepare the over coating layer having the resistance to printing, the hardwearing property and the resistance to scuffing in the practical use which is employed in the photoreceptor for electrophotography of the contact development system, the over coating layer having a thickness thicker than the coating film of the conventional plastics optical component is requested. However, a thick film prepared by employing a conventional thermosetting and photosetting paint is difficult to be obtained because the paint is contracted at the time of curing. The realization of the film thickness satisfying not only the electrical characteristics of the sensitive material or the electrophoto characteristics but also the high resistance to printing and the high resistance to scuffing is extremely difficult.